The present invention relates generally to millimeter and submillimeter wave amplifiers, and more particularly, to a traveling wave amplifier with a special wide-band frequency-spreading coupler for wide-band operation at high power levels.
Information carrying systems such as radar and other communications devices require an amplifier mechanism with substantial instantaneous bandwidth rather than simply an oscillation mechanism. In order to provide wide-band high power operation in traveling wave amplifiers, the use of a tapered interaction waveguide in conjunction with a specially profiled magnetic field has been proposed in Application Ser. No. 06/389,133, entitled "Wide-Band Gyrotron Traveling - Wave Amplifier" by Y. Y. Lau, L. R. Barnett, K. R. Chu, and V. H. Granatstein. The gyrotron traveling-wave amplifier disclosed therein comprises a tapered waveguide wherein the cross-section thereof gradually increases from a small first end to a large second end for propagating electromagnetic energy therein, a magnetron device for generating a beam of relativistic electrons with helical electron motion for application to the small first end of the tapered waveguide to propagate in the axial direction therein, a magnetic circuit for generating a tapered magnetic field within the waveguide in a direction approximately parallel to the axis of the waveguide, and an input coupler for launching an input elecromagnetic wave so that it co-propagates with the electron beam in the waveguide.
The above-mentioned waveguide is tapered such that its cutoff frequency varies over a predetermined bandwidth. This device then utilizes a reverse rf injection scheme wherein the electromagnetic wave to be amplified is applied at the large end of the tapered waveguide so that it propagates in the waveguide until its individual frequencies are reflected when they reach the point in the waveguide taper where they approximately match the cutoff frequency of the waveguide. These reflected frequencies then copropagate with and are amplified by the electron beam. The above described injection scheme utilizes a single port for the injection and extraction of radiation energy from the amplifier thereby requiring either a circulator or 3 dB hybrid junction device to separate the input and output radiation at the amplifier port. These separation devices limit the amplifier gain due to the impossibility of perfectly matching the components.
A distributed mode coupler has been disclosed in Application No. 389,132, entitled "Wide-Band Distributed rf Coupler" by L. R. Barnett. In that application an input waveguide is coupled by a plurality of channel frequency filters to the tapered interaction waveguide of the amplifier. The filters are positioned so that the interaction waveguide cutoff frequency approximately matches the wave frequency propagated by the filter thereby maximizing the gain in the amplifier. The coupler disclosed in Application No. 389,132 is a complex structure requiring the fabrication of precision channel filters. The bandwidth of the coupler is limited by the number of channels. Additionally, the distributed coupler does not have a continuous bandwidth profile since only those frequencies matched to a channel filter are coupled to the amplifier.